1. Field of the Invention
The present invention relates to a method and apparatus for monitoring or detecting amount of sub-micron particles every range of their dimensions.
The method according to the present invention is advantageously applicable to monitor and control impurity particles in fluid such as pure water and ultra pure water used in electronics industries, biotechnology, medical and pharmaceutical application and food industries. The method of the present invention can be used to evaluate the performance of separation membranes and filtration system.
2. Related Arts
The conventional method for monitoring or detecting particles in fluids are classified into following four categories;
(1) Shadow system in which decrement of light intensity caused by traveling particles in fluid passing across an optical axis of parallel ray.
(2) Microscope system in which fine particles in fluid are caught by a membrane filter or the like and are observed or counted by a electron scanning microscope.
(3) Light scattering in which fluid is irradiated with an intensive light such as a laser beam and the resulting scattered light is collected by a lens so that the focused light is detected by a photo-multiplier.
(4) Imaging system in which a fluid is irradiated with a light and the resulting contrast of light is detected by a photo-diode alley and an image of particles in the fluid id formed by a computer.
New techniques such as ultrasonic scattering techniques are also proposed.
In the case of the shadow system (1), however, direction of fine particles in limited to particle size of about 1 xcexcm and hence this detection system can not be used for sub-micron particles. In the microscope system (2), more than half day is required to obtain the result.
The light scattering system (3) is the main current of development in particle counters or detectors and now ultra-fine particles having the particle size of less than 0.07 xcexcm can be detected by using a light source having shorter wave length such as argon laser. In fact, Japanese patent laid-open No. 4-39,635 discloses a technique to determine the precise number of fine containment particles each having the particle size of lower than 0.07 xcexcm contained in ultra pure water. This patent proposes to use two detectors each receive the scattered light so that a particle counter produces a signal when two detectors detect the scattered light simultaneously. This system, however, requires a high-power laser as well as very sensitive photo-multiplier, resulting in a large costly system. Still more, in this system, precise alignment between an axis of fluid stream containing particles to be detected and an optical axis is required in order to assure the reliability of measurement. Japanese patent laid-open No. 62-803 discloses an automated apparatus that facilitates this alignment.
Japanese patent laid-open No. 63-19535 discloses a variation of the imaging system (4). In this patent, a laser beam impinges vertically to a flow of sample fluid and the diffracted and scattered light is passed through a Fourier-transformation optical system or a lens to produce a Fraunhofer diffraction image that is treated in order to evaluate fine particles in the fluid. In this patent, a diameter of a laser beam is enlarged to obtain a parallel ray that is directed to the sample fluid. This system requires a complicated computer system.
Then, for the above situation, we proposed a new method and apparatus for monitoring and detecting fine particles of sub-micron as contaminant in fluid by a simple and very economical apparatus as U.S. Pat. No. 5,534,999.
The above patent provides a method comprising converging a light beam from a coherent light source so that the resulting focused light passes through a stream of fluid containing particles therein in such a manner than a focus of the focused light is located in the stream of fluid, receiving a light passed through the stream of fluid and diffracted by the particles by a photo-detector which is positioned at an opposite side of the coherent light source with respect to the stream and on an optical axis of the light beam to produce electrical signals, and counting numbers of particles in the stream by treating the electrical signals.
The above patent provides also an apparatus comprising a coherent light source, an optical system for converging a light beam emitted out of the coherent light source to produce a converged light, a cell through which a stream of fluid containing particles is flown and being located in the neighborhood of a focus of the converged light beam, a photo-detector which is positioned at an opposite side of the coherent light source with respect to the stream and on an optical axis of the light beam to produce electrical signals, and an electric circuit for counting numbers of particles in the stream by treating the electrical signals.
The above method is based on such surprising and unexpected finding that the existence of sub-micron particles in a fluid stream can be detected or monitoring by utilizing diffraction phenomenon of a transmitted light, which is observed when a converged coherence light is focused on the light stream. In fact, it is not known to use the transmitted light of a converged light directly for detecting fine particle before the invention according to the patent.
In the method according to the above invention, the diffracted image can be obtained for a particle whose particle size is smaller than 0.1 xcexcm that is not observable in known techniques. Of course, particles having the particle size of bigger than 1.0 xcexcm also can be detectable with high sensitivity by the method according to the method.
In the conventional detection technique, dispersed particles are irradiated with an illumination parallel ray so that the resulting transmitted light image is Fourier-transformed as is described in the Japanese patent laid-open No. 63-19,535.
The detection principle of the method according to the present invention is different from those of known methods but the theory why the particles in fluid is detected precisely by the method according to the present invention can not be explained completely at this stage. Following is one of probable explanations.
By the way, the method and apparatus according to our patent can detect only amount of all particles existing in the fluid, but can not detect dimensions of the particles. Therefore, an object of the present invention is to provide an improved method and apparatus that permits to detect particles every range of dimensions of the particles.
As explained before, the method and the apparatus according to the invention disclosed in U.S. Pat. No. 5,534,999 permit detect an existence of very small particles fluid. Furthermore, according to the method and apparatus of the present invention, dimension of the particles can be detected.
In the method and the apparatus according to the invention disclosed in U.S. Pat. No. 5,534,999, the particles pass through the converged laser beam. Therefore, each passing time of the particle is different from each other corresponding to distance from the focal point of the converged light beam.
Namely, length of a passage of particles passing near by the focal point is very short. Length of a passage of the particles passing far from the focal point is very long. On the other hand, all the particle moves by equal velocity. Therefore, the photo-detector emits short signals corresponding to the particles passing near by the focal point, and the photo-detector emits a long signal corresponding to the particles passing far from the focal point. In other words, the signal frequency corresponds to the distance between the passage of the particles and the focal point.
At the same time, as explained before, small particles can be detected only passing near to the focal point. Therefore, low frequency signal emitted from the photo-detector corresponds to only large particles and high frequency signal emitted from the photo-detector corresponds to all particles.
Physical phenomenon mentioned above can be utilized for every range of dimension of the particle by electrically treating the signals emitted from the photo-detector.